Hitherto, in order to warn users of mothproofing agents, insecticides, deodorants, fragrance, and similar products that the term of efficacy of such chemicals is about to expire, there have been proposed various types of period indicators which display the quantity-related status of the liquid chemical.
For example, Japanese Patent Publication (kokoku) No. SHO 63-24961 or Japanese Utility Model Publication (kokoku) No. HEI 5-938 discloses a sheet structure having an indicative section and impregnated with a liquid chemical. The sheet structure includes a substrate formed of a chemical-liquid-permeable material, and, on the entirety or on a portion of one surface of the substrate, the indicative section formed from a chemical-liquid-permeable material having a low light refractive index. According to the disclosures of these publications, the chemical-liquid-permeable material having a low light refractive index for forming the indicative section employs, for example, amorphous silica. During the period in which the chemical is present, the indicative section remains transparent, whereas when the chemical has volatilized and been lost, the indicative section becomes opaque, whereby the user can know the final stage that the period of efficacy of the chemical has expired.
In recent years, there has arisen consumers' need for diversified modes of display, and to meet such need, there have been proposed indicators which can indicate not only the endpoint of the effective period but also the early stage or mid stage of the effective period.
Japanese Patent No. 3045651 discloses a period indicator having indicative portions on the surface of a substrate, wherein one indicative portion can show the final stage of the effective period and additionally another indicative portion can show an early stage or mid stage of the effective period. For showing that the final stage is approaching, an indicative layer having a low light refractive index is employed, whereas for showing that the product is in the early stage or middle stage of its service life, an indicative layer having a higher reflectance as compared with that of the substrate is employed. With this structure, since the display for showing the early stage or middle stage of use persists even when the final stage of service life, where the chemical is volatilized and lost, is approaching, measures must be taken so as mask the persisting sign with the sign showing the endpoint of the period, or alternatively, the persisting signs are left to show themselves but together with the sign showing the endpoint of the period. In either case, limitations are imposed on the letters or marks to be used for display.
Japanese Patent Application Laid-Open (kokai) No. HEI 8-106251 discloses use of a non-woven fabric as the chemical-liquid-permeable indicative material having a low light refractive index. It discloses that through use of a non-woven fabric whose density is partially changed, between the low density portion and the high density portion, change of color can be made to occur with a time lag, thereby permitting display of the level of volatilization of the chemical in a stepwise manner. Since this approach employs non-woven fabric, separate means such as printing onto the non-woven fabric is needed so as to display letters or marks.
As many functions for attaining diversified modes of display have been added, there have arisen needs for modification of the composition of the material that forms the indicative section, making production steps intricate and increasing costs.